All my Galley become invisible (like invisibility spell or skill) with an upkeep of 2 mana and now they have:
- Regeneration
- Illusion attack
- Create Outpost
- Corruption
- Instant Damage
- Destruction
- Fire Immunity
- and many more!

I play vanilla, no mod or save game editors.

Save game corruption. Be prepared for the save to become unusable; if you want to finish, to do so quickly before things degenerate further.

Any new game you start should be fine, but that one is teetering on the cusp of "Screwed".

Can an old save still be "healthy"? Or could it already be corrupted just not so clearly?
With insecticide pacth this will be less likely?

Yes, and yes. Depends on what caused the corruption, and how far back it stretches. There isn't a great way to know. Thankfully, v1.31, which is what GoG ships, is a lot more stable than the lesser patched versions, so corruption isn't the omnipresent threat it used to be.

I'm not sure how many straight bugs Insecticide patches, but it does change the game in some significant ways. There's a thread around here detailing insecticide changes.

Insecticide is outdated now, try 1.51R:
https://masterofmagic.fandom.com/wiki/Unofficial_Patch_1.50

Most likely, this is the result of a Raise Dead combat spell. This spell may be cast whenever, but the game is usually corrupted after the combat.

It doesn't matter who cast that, but after the combat, if you see a lot of invisible ships, it's a good idea to save in another saveslot and restart the game. If it solves the issue, alright. Else, load a save from before the combat (you do save before every combat, right? Especially if you cast those game-corrupting spells: Raise Dead, Vortex, etc).

There are other sources of game-corruption, but Raise Dead game-corruption is corrected by a reload and usually doesn't corrupt the save game.